Add conntection variables support to \copy

[ejiektpobehuk]

This PR addresses #495 and similar issues I've experienced.

\copy had no support for connection variables set with \cset or a configuration file.
It went straight to parsing the source and destination with dburl.Parse().

I've copied the Handler.Open() behaviour without any DRY.
Now it can handle connection variables.

[ejiektpobehuk]

I'd like to add a test to the test suite. My current version is file system dependant and I'm not sure if it's a good fit. On the other hand this test uses csvq and sqlite3 which are built-in and don't require external databases.

-- Create source data
\! mkdir -p /tmp/usql_test
\! echo "id,name,value" > /tmp/usql_test/source.csv
\! echo "1,Alice,100" >> /tmp/usql_test/source.csv
\! echo "2,Bob,200" >> /tmp/usql_test/source.csv
\! echo "3,Charlie,300" >> /tmp/usql_test/source.csv

-- Set up connection variables using \cset
\cset SOURCE csvq:///tmp/usql_test
\cset DEST sq:/tmp/usql_test/dest.db

-- Create destination table
\c DEST
CREATE TABLE test_table (
  id INTEGER PRIMARY KEY,
  name TEXT,
  value INTEGER
);

-- Test target
\copy SOURCE DEST 'SELECT * FROM `source.csv`' 'test_table'

-- Verify the data was copied
\c DEST
SELECT * FROM test_table;

-- Cleanup
\! rm -rf /tmp/usql_test

\quit

[ejiektpobehuk]

Test results before the patch:

CREATE TABLE
error: invalid database scheme
(0 rows)

Test results after the patch:

CREATE TABLE
COPY 3
 id |  name   | value
----+---------+-------
  1 | Alice   |   100
  2 | Bob     |   200
  3 | Charlie |   300
(3 rows)